Method of and means for hydrodynamic mixing

ABSTRACT

A plurality of fluent substances are combined in a distributor (11, 37) and caused to flow in a continuous, pressurized stream through a mixing zone (13, 13&#39;, 57) wherein the substances are intimately mixed by turbulent dispersion effected by a series of conically shaped surfaces (24, 27; 62, 64) and alternate restrictions (25, 63) and expansion chambers (28, 65). While various fluent substances may be advantageously mixed, a typical utility is for effecting a through air binding of ink particles to attain efficient foaming (29) in the deinking of reconstituted printed paper slurry.

DESCRIPTION

This invention relates to hydrodynamic mixing and is more particularlyconcerned with mixing of a plurality of fluent substances which arerequired to be combined into an intimate mixture for a processingsystem, such as, but not limited to, paper making where one of thesubstances may be fibrous stock in a slurry and another of thesubstances may be gaseous finely particulate material which must beintimately mixed with the fibrous stock.

By way of example, a considerable problem has been encountered inattaining efficient, uniform results in deinking reconstituted printedpaper slurry. Such deinking can be effected by intimately mixing air inthe slurry and allowing air bubbles to engage with as many ink particlesas possible. The ink particles attached to the air bubbles aresubsequently separated in a floatation cell where the air bubbles carrythe ink to the surface and the ink is removed with the foam thatdevelops on the surface.

In order to enhance the mixing, it is important to have an evendistribution of air into the fiber suspension even before the mixingstarts. The purpose of the mixing is to increase the probability for theair bubbles to meet and pick up ink particles.

Turbulence for mixing the air in the slurry has heretofore been effectedby means of a plurality of perforated disks in the air/slurry stream.While this arrangement has functioned adequately in some situations,when long fibers are present in the slurry, clogging tends to resultaround the disks. Furthermore, while turbulence was created immediatelyaround each disk, and since the area between disks created no influenceon the flow, the turbulence had a tendency to decay significantly beforethe flow reached the next disk. The fibers in the stock tend toreinforce laminar flow, thus inhibiting mixing and further enhancingdecay of turbulence. Stalling or slowdown of the stream has resulted inagglomeration of the air bubbles.

Any screens in the system, such as may be employed at the point wherethe air is added to the slurry, tend to clog and thus diminishefficiency.

In another example where intimate mixing of one fluent substance withanother fluent substance is necessary is in bleaching operations whereingases, such as oxygen and ozone, are mixed with fluent material such aspaper making stock. It will be evident that in order to attain efficientresults, the bleaching substances and the substances to be bleached mustbe uniformly intimately mixed.

An important object of the present invention is to overcome thedisadvantages, drawbacks, inefficiencies, limitations, shortcomings andproblems inherent in the prior expedients for mixing, and maintainingmixed, fluent substances in a continuous flow system.

Another object of the invention is to provide a new and improved methodand apparatus for attaining intimate mixing of flowing gaseous andparticulate substances with a liquid.

A further object of the invention is to provide a new and improvedmethod of and means especially suitable for attaining efficientturbulent aeration of recycled ink-containing paper making fibrousslurry previous to a stage for removal of the ink particles carried byair bubbles.

Still another object of the invention is to provide a new and improvedmethod of and means for thoroughly and evenly aerating paper makingslurry in a continuous flow processing system.

An additional object of the present invention is to provide a mixingdevice which exerts continuous influence on the flow stream, therebyeliminating the tendency for decay of turbulence heretofore experiencedwith fibrous stock.

A still further object of the present invention is to provide a mixingdevice which creates highly turbulent mixing in a smooth, continuous,efficient manner.

Yet another object of the present invention is to provide a mixingdevice which generates three-dimensional mixing.

Pursuant to the principles of the present invention there is provided amethod of attaining an intimate mixture of a plurality of fluentsubstances in a continuous flowthrough passage having an entry end and adischarge end, and comprising combining the plurality of fluentsubstances at the entry end in a continuously flowing stream filling theflowthrough passage under substantial hydrodynamic pressure, subjectingthe stream in the passage to the turbulence and substance dispersingeffect of a series of alternating radially inwardly tapering relativelyshort conically shaped turbulence surfaces and radially outwardlyflaring longer conically shaped turbulence surfaces, effecting abruptturbulent transition of the stream from one of the surfaces to the nextof the surfaces in the series, thereby attaining progressively morethorough dispersion and mixing of the substances in the continuouslyflowing stream from the entry end to the discharge end of the passage,and discharging the thus treated stream from the discharge end of thepassage to receiving means.

There is also provided a new and improved apparatus for practicing themethod. The apparatus includes a distributor for combining fluentsubstances into a stream under substantial hydrodynamic pressure. Amixing zone in the apparatus receives the combined substances stream andis essentially open with regular smooth turbulence effecting surfaces,with no fiber catching areas, and is arranged to exert a continuous andprogressive dispersing and mixing turbulence in the flow pattern of thestream.

Other objects, features and advantages of the invention will be readilyapparent from the following description of representative embodimentsthereof, taken in conjunction with the accompanying drawings, althoughvariations and modifications may be effected without departing from thespirit and scope of the novel concepts embodied in the disclosure, andin which:

FIG. 1 is a schematic illustration wherein the present invention isembodied in a system for deinking reconstituted printed paper slurry.

FIG. 2 is an enlarged fragmentary sectional detail view takensubstantially along the line II--II in FIG. 1;

FIG. 3 is a sectional detail view taken substantially along the lineIII--III in FIG. 2;

FIG. 4 is a fragmentary vertical sectional detail view similar to FIG.2, but showing a modification of the distributor at the entry end of themixing zone passage of the apparatus;

FIG. 5 is a transverse sectional view taken substantially along the lineV--V in FIG. 4; and

FIGS. 6 and 7 disclose a modified construction of the turbulence modulesfor the mixing zone of the apparatus.

A processing system is schematically shown in FIG. 1, which isespecially adapted for deinking a reconstituted printed paper fiberstock slurry for making fresh paper. A continuous stream of the slurryis delivered through a supply pipe 10 into an air distributor 11 locatedat the upstream or entry end of a flowthrough passage 12 extendingthrough means defining a mixing zone 13. At its downstream or dischargeend, the passage 12 discharges into an air separation cell 14.

In one preferred construction, the distributor 11 comprises a ringshaped member 15 (FIGS. 2 and 3) providing a central circular aerator orcombining chamber 17 into which a fluent substance such as a slurry ofpaper making stock is delivered axially by the supply pipe 10. The otherfluent substance, such as air or any other desired gas or fluid, to bemixed with the slurry supplied through the pipe 10, is injected into thechamber 17 generally tangentially at the cylindrical inner diameter ofthe ring 15 through a port 18 to which is connected a delivery line 19.Thereby, the substance delivered by the line 19 is injected in an evendistribution into the chamber 17 and swirls about and into the stream ofmaterial entering the chamber 17 from the pipe 10.

From the combining chamber 17, the comingled gas, i.e., air, and slurrystream enters the passage 12 in a continuous flow filling the passageunder substantial hydrodynamic pressure. As the stream enters andtravels through the mixing zone 13, the stream is subjected to repeatedand progressively effective substance dispersion and mixing. In apreferred arrangement for this purpose, the mixing zone 13 is housedwithin an elongated cylindrical tubular casing 20 to the upstream orentry end of which the supply pipe 10 is attached in any suitablemanner, either integrally as shown in FIG. 2, or by means of any otherappropriate hydraulic connection.

Housed within the mixing zone 13 portion of the casing 20 are meanscomprising a series of mixing modules 21 in end-to-end cooperation. Eachof the modules 21 is desirably of a substantially standardizedconstruction and comprises a cylindrical body which may be slidablyreceived within the casing 20. This permits the modules 21 to be easilyinterchangeably assembled within the casing 21 and to be replacedwhenever desired. Each module 21 has an upstream end provided with anarrow annular axially facing outer diameter abutment shoulder 22 whichis engaged in the assembly with a complementary oppositely axiallyfacing abutment shoulder 23 at the downstream end of a companion module21. In other words, each of the modules 21 has an upstream end abutmentshoulder 22 and a downstream end abutment shoulder 23. In order toaccommodate and match with the engaged surface of the distributor ring15, the upstream end, or first, module 21 in the series has an upstreamend abutment shoulder surface 22a which may be wider than thecorresponding shoulder surfaces 22 of the remaining modules 21.

Extending from the radially inner edge of the upstream end shoulder 21of each of the modules 21 is an annular frustoconical generally radiallyand axially inwardly extending, relatively narrow, funnellike turbulencesurface 24 terminating at an abrupt transition edge 25 at the upstreamedge of a substantially longer generally axially and radially outwardlyextending conically shaped, i.e., frustoconical, surface 27 whichextends to the downstream end shoulder 23 of the module 21.

In one preferred construction, where the outside diameter of the module21 has been about 75 mm, desirable dimensions for the surface 24 havebeen about 68 mm in the major or outside diameter and about 30 mm in theminor diameter at the abrupt transition or restriction edge 25. Ashallow diagonal angle of about 15° in the surface 24 relative to thediameter of the module 21 has been found desirable. Where the length ofthe module 21 is about 75 mm a shallow cone angle of about 15° relativeto the cone axis for the surface 27 has been found desirable. Therelative length of the surfaces 24 and 27 may be about one to four. Atthe abrupt juncture 25, the surfaces 24 and 27 are related insubstantially right angular relationship. The right angular relationshipis also apparent where the widest end of the surfaces 27 join the widestend of the surfaces 24. The arrangement is such that the area of achamber 28 within each of the modules 21 progressively increases fromthe narrow flow restriction entrance at the restriction edge 25 to anabout five times larger cross sectional flow area in the maximum crosssectional area at the exit end of the chamber 28. While the five timeslarger flow area ratio at the exit end of the chamber relative to theentrance end at the restriction is preferred for certain types of papermaking stock of a given consistency, such ratio may be from two to oneup to eight to one, depending upon the type of stock and especially thestock consistency because the consistency controls the fiber networkstrength and thus the power required to break up the fiber network,i.e., create turbulent mixing by fluidization.

As will be observed, there are no surfaces throughout the length of thepassage 12 within the mixing zone 13 on which fibers would tend to hangup or be retarded in movement with the stream. Therefore, although thefibers in the stream, such as the reconstituted paper stock fibersreferred to, might tend to reinforce flow and thus inhibit mixing, ithas been found that the arrangement of abrupt restriction followed by agradual expansion leading to another abrupt restriction creates a highlyturbulent flow without standing eddies or other undesirable flowpatterns.

In operation of the mixing zone 13, the stream of combined fluentsubstances entering the mixing zone 13 from the distributor chamber 17is continuously influenced by the modules and is repeatedly subjected toa substance mixing three-dimensional turbulence as the stream progressesthrough the mixing zone, attaining progressively more thoroughdispersion and mixing of the substances in the continuously flowingstream in the passage 12. At the discharge end of the passage 12, thematerial is discharged into the flotation unit 14 as a uniform mixture.

Tracing the progress of the stream through the mixing zone 13, at theentry end the material from the distributor 11 is subjected to themixing action of an initial agitation on striking the first turbulencesurface 24 then accelerated on passing the first abrupt restriction edge25, followed by turbulent pressure drop in the chamber 28 of the firstmixing module 21 in the series. This vigorous mixing action is repeatedas the stream flows through each of the successive modules 21,efficiently reaching a high degree of uniformity in the mixture by thetime the mixture leaves the mixing zone 13. Inasmuch as turbulence isgenerated without using extremely narrow restrictions or obstructions,clogging by fibers of the stock in the stream is not a problem. Thepipe-like shape generates severe turbulence in three dimensions andmakes the mixer surfaces self-cleaning.

Depending upon the nature of the fluent substances to be mixed, theremay be any desired plurality of the mixing modules 21 in the series,five modules being shown for attaining a high degree of mixing of air orother gas with a slurry which requires multiple agitations in successiveturbulence stages throughout the mixing zone for attainment of thedesired mixture uniformity. At each agitation or turbulence stage as thestream progresses through the mixing zone 13, there is a progressivelymore intimate mixing of the substances, e.g., air and fibers, in therapidly flowing stream. Mixing is promoted by the stream striking eachof the turbulence wall surfaces 24 and thereby breaking up any tendencytoward channeling, in spite of the flow reinforcing tendencies of thefibers, and also promoting turbulence, as the stream is diverted towardthe restriction 25. In the stream velocity through the restriction 25and the abrupt transition to the flaring surface 27 and pressure dropand controlled uniform expansion toward the maximum area of the chamber28, turbulent mixing progresses in each module 21. As the turbulence andmixing cycle is aggressively repeated in each stage in the mixing zone13, the mixture reaches maximum yield at final discharge into thereceiving chamber of the separation cell 14 wherein air bubbles with inkattached rise as a foam 29 to the surface of the body of agitated slurry30. A vacuum drawoff 31 removes the ink laden foam 29. The velocity ofentry of the thoroughly mixed air and slurry into the cell 14 iscontrolled to assure that as a result of the boyancy force and liquidvelocity the ink carrying bubbles will be efficiently drawn off in thefoam 29. The cleaned fiber slurry flows past a foam baffle 32 and spillsover a dam 33 into a discharge chamber 34 from which a drain pipe 35carries the slurry to a further processing point or to another deinkingstage if necessary, screening, or other processing as may be desired.

For some purposes a distributor 37 as shown in FIGS. 4 and 5 may bedesirable. In this arrangement, a plurality of fluent substances iscombined in a continuously flowing stream completely filling aflowthrough chamber 38 within a cylindrical elongated housing 39 havingan end closure 40 at the upstream or entry end of the chamber 38.Adjacent to the closure 40, an inlet 41 discharges preferably theheavier of the substances to be mixed, such as paper making stockslurry, tangentially through the wall of the housing 39 into the maximumcross sectional full volume area of the chamber 38. Axially through theend closure 40, an inlet pipe 42 provides a nozzle 43 by which anothersubstance such as air or any other desired gas or substance is injectedaxially into the chamber 38. Centered in the discharge port of thenozzle 43 is a tip 44 of a flaring conical surface 45 along which aconical film or layer of the substance from the nozzle 43 travelsenveloped in the generally spirally moving stream of the substance whichhas been delivered by the inlet 44 into the chamber 38. The combinedstreams flow in spiral fashion downstream along the flaring surface 45to an annular abrupt transition edge 47 defining with the housing wall39 a venturi orifice 48. At the venturi orifice 48, the substancetraveling along the surface 45 is driven at high velocity into thesubstance in the stream which has been delivered into the chamber 38through the inlet 41. At the downstream end of the orifice 48, thecombined stream is turbulently ejected with efficient substance-mixingeffect into a space 49 connected with the upstream end of a mixing zone13' which is similar to the mixing zone 13 of FIG. 2, into which theturbulently moving stream is delivered in essentially the same manner asthe combined stream is delivered from the comingling chamber 17 in FIG.2. Within the mixing zone 13', the modules 21' may, if preferred, beshorter, as shown, but in other respects substantially the same as themodules 21 in FIG. 2.

In a desirable construction, the conically shaped surface 45 may beprovided on a conical member 50 having a radially and axially inwardlytapered downstream end surface 51 joining a central bullet nose stem 52projecting in downstream direction and attached to means such as aradially outwardly extending supporting fin structure 53 provided withgenerally upstream knife edges 54, sloping generally radially andaxially inwardly substantially parallel to the surface 51, and locatedin the area of the space 49 which receives the mixture stream in itsmost turbulently agitated condition downstream adjacent to the orifice48. Freedom from hangup of fibrous material is thereby assured.

For some mixtures, shorter turbulence promoting stages of larger numbermay be desirable. This may be effected by having the individual modulesshorter as shown in FIG. 4 for a given length of mixing zone. On theother hand, a cylindrical tubular housing 55 similar to the describedhousing 20, may define a mixing zone 57 having therein a series ofturbulence generating modules 58 each of which may be of about the samelength as the modules 21 in FIG. 2, but each being of multi-stagestructure and having, as shown but not limited to, two turbulencegenerating stages 59. At their upstream and downstream ends, the dualmodules 58 have respective axially facing annular surfaces 60 and 61which abut the respective opposing end surfaces of the contiguousmodules 58 in the series. Each of the stages 59 has a relatively shortradially inwardly and axially downstream slanted generally conicalturbulence surface 62 joining on an abrupt restriction edge 63 arelatively longer generally conical radially outwardly and downstreamflairing turbulence surface 64 of desirably about the same angularity asthe surfaces 24 and 27, but respectively shorter than those surfaces.The diameter of the restrictions 63 is substantially greater than therestrictions 25 and, the minimum diameter of a through passage 65 in themixing zone 57 is greater than the minimum diameter of the throughpassage 12. The downstream end of each of chambers 67 in the modulesections 59 may be of the same diameter as the downstream end of thechambers 28.

Since the geometrical configuration of the modules 58 is substantiallythe same as the modules 21, but in a plural mode, the stream agitatingturbulence effect in the multiple stages 59 along the modules 58 occurssubstantially the same as described for the modules 21, but withincreased frequency, and with possibly less intensity where that ispermissible for a given hydrodynamic pressure. However, mixing intensitymay be compensated for, where desired, by increased velocity in thestream through the passage 65.

By virtue of the smooth, circular surfaces and their cooperation in themixing zones 13, 13' and 57 provided by the present invention, evenwhere some of the surfaces are relatively abrupt, there is attained notonly assured intimate mixing of substances in the hydrodynamic streamsof material in the passages through the mixing zones, but also freedomfrom pocketing or fiber hang up of the flowing material. It may also benoted that the mixing zones are free from moving parts, and are ofmodular construction providing for simplicity not only for manufacturingconvenience, but also for ease and convenience of assembly and changingor replacement of the mixing modules for any desired changes in mixingintensities. This contributes in the present invention to a method andmeans of high efficiency and low cost.

Although use of the present invention in connection with deinkingreconstituted paper slurry has been selected as a principal example,other uses will be obvious. For example, the invention may be used forpulp slurry deflocculation ahead of a head box in a paper makingmachine. Other types of mixing may also be effected, such as mixingdifferent types of pulp slurries, mixing pulps and chemicals, and thelike.

It will be understood that variations and modifications may be effectedwithout departing from the spirit and scope of the novel concepts of thepresent invention.

We claim:
 1. A method of attaining an intimate mixture of a plurality offluent substances in a continuous flow-through passage having an entryend and a discharge end, and comprising:combining said plurality offluent substances at said entry end in a continuously flowing streamfilling said flow through passage under substantial hydrodynamicpressure; subjecting said stream in said passage to turbulence andsubstantial dispersion and mixing effect from a series of adjacentalternating radially inwardly tapering relatively short frustoconicalturbulence surfaces providing acceleration and radially outwardlyflaring relatively longer frustoconical turbulence surfaces providinggradual dispersion; effecting abrupt turbulent transition of the streamfrom each of said surfaces to the next of said surfaces in the series;influencing the stream continuously between the entry end and thedischarge end by one of gradual expansion and dispersion from theoutwardly flaring surfaces, relatively more rapid acceleration andrestriction from the inwardly flaring surfaces, and abrupt transitionbetween expansion and restriction; thereby attaining progressively morethorough dispersion and mixing of said substances in the continuouslyflowing stream from said entry end to said discharge end of saidpassage; and discharging the thus treated stream from said discharge endof the passage to receiving means.
 2. A method according to claim 1,which comprises supplying one of said substances as a fibrous papermaking slurry, and combining another of said substances as a gas withsaid slurry.
 3. A method according to claim 2, wherein said slurrycomprises reconstituted printed paper, and supplying said gas in theform of air to said slurry for combining with ink particles as a resultof said dispersion and mixing.
 4. A method according to claim 3, whichcomprises discharging the treated stream into an ink separation cell.